1. Technical Field
The present invention relates to a linear power amplifier, and more particularly, to a linear power amplifier and method able to minimize intermodulation distortion with a predistortion technique and feed forward technique.
2. Background Art
High power amplifiers (HPA) are used in a wide variety of communications and other electronic applications. High power amplifiers typically operate in the vicinity of a saturation region having non-linear characteristics in order to generate a maximum output. However, we have noticed that if more than one carrier, i.e., a multi-carrier is applied to the high power amplifier, its non-linear characteristics introduce unwanted multiplicative interaction of the signals being amplified and the amplifier output contains intermodulation distortion (IMD). Intermodulation distortion is highly undesirable because it causes interference, crosstalk, and other deleterious effects on the performance of a system employing the high power amplifier. Generally, intermodulation distortion can be reduced by either negative feedback of the distortion components, predistortion of the signal to be amplified to cancel the amplifier generated distortion, or by separating the distortion components of the amplifier output and feeding forward the distortion components to cancel the distortion in the amplifier output signal.
Conventional negative feedback techniques for reducing intermodulation distortion produced by high power amplifiers such as disclosed in U.S. Pat. No. 4,929,906 for Amplifier Linearization Using Down/Up Conversion issued to Voyce, and U.S. Pat. No. 5,237,288 for RF Power Amplifier Linearization issued to Cleveland, generally seek to feed the power amplifier's output back to a differencing or error stage for intermodulation distortion reduction. We have found that such feedback systems are intended for low frequency applications and do not take into account RF design considerations such as time delay and drift. Nevertheless, feedback system are preferable for RF power amplifiers because they are closed loop systems that are less susceptible to changes caused by aging and environmental factors.
Efforts to implement predistortion techniques for reducing intermodulation distortion such as those disclosed in U.S. Pat. No. 4,967,164 for Adaptive Predistortion Circuit issued to Sari, U.S. Pat. No. 5,523,716 for Microwave Predistortion Linearizer issued to Grebliunas et al., and U.S. Pat. No. 5,524,286 for Baseband Predistortion System For The Adaptive Linearization Of Power Amplifiers issued to Chiesa et al., generally utilize an auxiliary distortion source that produces an auxiliary distortion signal similar to the distortion generated by the power amplifier. The auxiliary distortion signal is added to the power amplifier input in the correct gain the phase to to promote cancellation of the distortion at the power amplifier's output.
Feed forward techniques such as those attempted by U.S. Pat. No. 3,922,617 for Adaptive Feed Forward System issued to Denniston et al., U.S. Pat. No. 4,885,551 for Feed Forward Linear Amplifier issued to Myer, U.S. Pat. No. 5,130,663 for Feed Forward Amplifier Network With Frequency Swept Pilot Tone issued to Tattersall, Jr., U.S. Pat. No. 5,166,634 for Feed-Forward Amplifier issued to Narahashi et al., U.S. Pat. No. 5,304,945 for Low-Distortion Feed-Forward Amplifier issued to Myer, U.S. Pat. No. 5,327,096 for Control Circuit For Automatically Controlled Feed Forward Nonlinear Distortion Compensation Amplifier issued to Sakamoto et al., is and U.S. Pat. No. 5,455,537 for Feed Forward Amplifier issued to Larkin et al., are more advanced in seeking to separate the distortion generated by a power amplifier and add it back into the power amplifier's output with gain, phase and delay adjusted for maximum cancellation. We have found that the amount of reduction in distortion obtainable using feed forward is primarily limited by the accuracy of the gain and phase adjustments. In typical feed forward amplifier designs previously attempted, for example, in Denniston '617, Myer '551, Tattersall '663 and '022, Narahashi '634, and Larkin '537, a pilot signal is combined with an input signal. The power amplifier then amplifies both signals and introduces non-linear amplitude and phase distortion into both signals during the amplification process. The amplified signal is sampled and the sampled input signal is subtracted from the sampled amplified signal to extract the distortion in the sampled amplified signal. The extracted distortion which is known as an error signal, is then adjusted in amplitude and phase based upon the level of the pilot signal detected at the output of the feed forward amplifier network. The adjusted error signal is amplified and subtracted from the originally amplified signal to produce a corrected signal having less distortion than the originally amplified signal. Thus, these types of feed forward systems endeavor to reduce the distortion introduced by the high power amplifiers with a reduction in the level of the pilot signal in the power amplifier's output. The pilot signal is used to monitor the level of non-linear distortion contained in the corrected signal and to continuously suppress the phase and gain of the error amplifier regardless of design factors. We have found however, that complex and costly circuitry such as a pilot tone generator and a pilot tone detector are required to enable the amplifier to generate and receive the pilot signal.
While there are other designs for high power amplifiers that seek to reduce intermodulation distortion in an ouput signal without using a pilot system such as that recently disclosed in U.S. Pat. No. 5,491,454 for Method And Apparatus For Reducing Distortion In An Output Signal Of An Amplifier issued to Matz, we have observed that a need still exists for an improved linear power amplifier that can reliably effectively remove intermodulation distortion in an output signal using a relatively simple circuit.